The present invention relates to printed circuit boards, and more particularly, to a printed circuit board formed from two printed substrates.
A prior art printed circuit board is as shown in FIG. 1. A printed substrate 1 has conductive foils 1b and 1b formed on both surfaces of a single insulating substrate 1a made of a paper epoxy resin substrate or glass epoxy resin substrate. The substrate 1a is provided with a hole 2 therethrough, and a chip component 3 such as a chip capacitor is inserted through the hole 2. Electrode portions 3a and 3a at ends of the chip component 3 are soldered to the conductive foils 1b and 1b, as shown by numeral 4.
Another type of circuit board is shown in FIG. 2, wherein printed substrates 1 and 1 each having a conductive foil 1b formed on the outer surface of insulating substrate portion 1a are placed one over the other, and bonded to each other by a binder 5 made of a thermosetting resin. A chip component 3 is inserted into aligned holes 2 provided in both substrate portions 1a and 1a. Electrode portions 3a and 3a of the chip component 3 are soldered at 4 to the conductive foils 1b and 1b.
In general, the chip component 3 is constructed as shown in FIG. 3. A rod-like or cylindrical insulating body 3b is made of a ceramic material, and its surface has a film 3c of a resistor material or the like formed thereover. The electrode portions 3a and 3a are disposed at both ends of the insulating body 3b. An insulating coating 3d is formed so as to cover the film 3c which is located between the electrodes 3a and 3a.
When such chip component 3 is inserted in the hole or holes 2 of the printed substrate or substrates 1 and soldered to the conductive foils 1b as illustrated in FIGS. 1 or 2, the insulating body 3b of the chip component 3 and the insulating substrate 1a of each printed substrate 1 may expand due to thermal effects. After the soldering, they may each shrink due to cooling. Further, when an electric equipment having such a printed circuit board is operated, the insulating body 3b and the insulating substrate portion 1a may repeatedly expand and contract with the rise and fall of the temperature within the electric equipment. The insulating body 3b of a ceramic material and the insulating substrate portion 1a of the paper epoxy resin substrate or the like typically have unequal coefficients of thermal expansion, and the differential thickness of the insulating substrate portion 1a between the expanded and contacted states thereof is several times greater than that of the insulating body 3b in the direction of the hole 2. For this reason, a stress acts on the soldered joint between the chip component 3 and the printed substrate 1. The stresses produced by the expansion and contraction of the printed substrate 1 may damage the conductive foil 1b and the electrode portion 3a or cause the conductive foil 1b or the electrode portion 3a to separate from the soldered joint 4. Such known circuit boards therefore have the disadvantage that the chip componet 3 cannot be connected reliably to circuit patterns on the board.